Prior Art
It has been widely known that the directional stability of a motor vehicle is lost in some cases, depending on the conditions of the road surface, if the wheels of the motor vehicle are locked when the brake is applied sharply. In view of this, it is in practice to control the braking force in such a manner as to eventually hold the slip factor of the wheels at approximately 20%, that is, so as to obtain the maximum coefficient of friction, by controlling the brake fluid pressure in accordance with the changes in the wheel velocity and the wheel acceleration, in consideration of the fact that the wheel velocity declines abruptly immediately before the coefficient of friction .mu. attains the maximum value in relation to the wheels when the pressure of the brake fluid working on the wheel cylinder is increased at the time of braking.
Specifically, motor vehicles are provided with an anti-skid control system which controls the braking force by finding the estimated vehicle speed on the basis of the rotating speed of the wheels, namely, the wheel velocity, at the time of a braking operation and the acceleration of the motor vehicle and then reducing, augmenting, or maintaining the pressure of the brake fluid working on the wheel cylinder in accordance with the difference between the two factors mentioned above, which occurs along with the changes in the wheel velocity along with the passage of time, and the acceleration of the wheels, so that the wheels are not locked at the time of sharp braking.
Such an anti-skid control system is applied either to the control of the front and rear wheels, namely, all the wheels, or to the control of the rear wheels, and, in either of these applications, the system needs wheel velocity sensors which detect the rotating speed of at lease the wheels to which the driving force is transmitted, that is, the driving wheels. For the control of the front and rear wheels, such rotating speed sensors are installed for all the individual wheels including the driven wheels, and, for the control of the rear wheels, such a construction would be feasible. Yet, in consideration of various advantages, such as the simplicity of construction, wheel velocity sensors are arranged only on the side of the rear wheels, which are driving wheels.
In this regard, it is noted that a wheel spin occurs at the time of a start or an acceleration on a road surface with a low coefficient of friction, such as a snow-covered road, if excessive driving force is transmitted to the wheels. That is to say, slips will occur not only at the time of braking applied to a motor vehicle, but also at the time of its acceleration, in what is generally called "acceleration slip". In this instance, a system which is capable of detecting also the wheel velocity of the front wheels, which are the driven wheels, for example, the system embodying the technology disclosed in the Official Gazette for Patent Laid Open No. 22551-1985, can detect the acceleration slip of the driving wheels. However, in case wheel velocity sensors are installed only for the rear wheels, the detected wheel velocity will be higher than the actual vehicle speed, and it is feared that the brake fluid pressure control based on the estimated vehicle speed as mentioned above will be inadequate. In contrast with this, a method of developing a simulated vehicle speed, which makes it possible to following up on the wheel velocity up to a point immediately preceding a braking operation, is laid open, for example, in the Official Gazette for Patent Publication No. 36111-1980. Specifically, the system is designed to cause the capacitor to perform an electric discharge in accordance with the increase of the output from the acceleration sensors after the rapid discharge circuit is cut off as the brake switch is turned on.